Correcting Non-Linear Loudspeaker Response

Abstract

The invention relates to processing an audio signal to correct the non-linear frequency response of a loudspeaker. An input audio stream ( 1801 ) is received, from which an input frame ( 1802 ) is generated. The input frame is subjected to a Fourier transform thereby creating a frequency spectrum defining magnitude and phase values for a plurality of frequency bands. The frequency spectrum is adjusted by scaling the magnitude and phase values of each of the plurality of frequency bands by a magnitude and phase correction coefficient. Dither may be added. An output frame ( 1806 ) is then created by performing an inverse Fourier transform on the frequency spectrum, so as to create a portion of an output audio stream ( 1809 ).

What we claim is: 
     
         1 . A method of processing an audio signal to correct the non-linear frequency response of a loudspeaker, the method comprising steps of:
 receiving an audio signal as an input audio stream comprising digital samples;   generating an input frame from the input audio stream, comprising a plurality of said digital samples;   performing a Fourier transform on the input frame, thereby creating a frequency spectrum defining magnitude and phase values for a plurality of frequency bands;   adjusting the frequency spectrum by scaling the magnitude and phase values of each of the plurality of frequency bands by a magnitude and phase correction coefficient;   performing an inverse Fourier transform on the adjusted frequency spectrum to create an output frame;   outputting the output frame as part of an output audio stream.   
     
     
         2 . The method of  claim 1 , further comprising a step of applying a windowing function to the input frame prior to performing the Fourier transform. 
     
     
         3 . The method of  claim 1 , wherein the magnitude and phase correction coefficient for a particular frequency band is defined in a lookup table. 
     
     
         4 . The method of  claim 3 , wherein:
 a plurality of lookup tables are available, each one of which corresponds to a possible volume level for the audio signal, and   the magnitude and phase correction coefficient for each frequency band is selected from the lookup table corresponding to a selected volume level for the audio signal.   
     
     
         5 . The method of  claim 3 , wherein:
 a plurality of lookup tables are available, each one of which corresponds to a possible volume level for the audio signal, and   the magnitude and phase correction coefficient for each frequency band is selected from the lookup table corresponding to an effective volume level for the audio signal, wherein the effective volume level is derived by scaling the selected volume level by the magnitude of the frequency band being adjusted.   
     
     
         6 . The method of  claim 1 , wherein the outputting step involves combining a plurality of output frames using an overlap add procedure to create the output audio stream. 
     
     
         7 . The method of  claim 1 , further comprising a step of applying a degree of dither to the magnitude and phase values in the frequency spectrum. 
     
     
         8 . The method of  claim 1 , wherein the magnitude and phase correction coefficients flatten the frequency response of the loudspeaker. 
     
     
         9 . The method of  claim 1 , wherein the magnitude and phase correction coefficients extend the effective bandwidth of the loudspeaker. 
     
     
         10 . A non-transitory computer-readable medium encoded with computer-readable instructions that, when executed by a computer, cause the computer to perform a method of processing an audio signal to correct the non-linear frequency response of a loudspeaker, the method comprising steps of:
 receiving an audio signal as an input audio stream comprising digital samples;   generating an input frame from the input audio stream, comprising a plurality of said digital samples;   performing a Fourier transform on the input frame, thereby creating a frequency spectrum defining magnitude and phase values for a plurality of frequency bands;   adjusting the frequency spectrum by scaling the magnitude and phase values of each of the plurality of frequency bands by a magnitude and phase correction coefficient;   performing an inverse Fourier transform on the adjusted frequency spectrum to create an output frame;   outputting the output frame as part of an output audio stream.   
     
     
         11 . An apparatus for correcting the anticipated distortion of an audio signal by a loudspeaker having a non-linear frequency response, the apparatus comprising:
 an input frame generator, configured to receive an input audio stream comprising digital samples and generate input frames each comprising a plurality of samples;   a Fourier transform processor configured to subject each of said input frames to a Fourier transform, thereby creating frequency spectra of the input frames, each frequency spectrum defining the magnitude and phase values of each of a plurality of frequency bands;   a storage device having stored therein a set of magnitude and phase correction coefficients, each one of which corresponds to one of the plurality of frequency bands;   a multiplier configured to scale the magnitude and phase values of each frequency band in each of the frequency spectra by the corresponding magnitude and phase correction coefficient;   an inverse Fourier transform processor configured to perform an inverse Fourier transform on the output of the multiplier to create output frames; and   an output frame combiner configured to combine said output frames to generate an output audio stream.   
     
     
         12 . The apparatus of  claim 11 , wherein the magnitude and phase correction coefficients flatten the frequency response of the loudspeaker. 
     
     
         13 . The apparatus of  claim 11 , wherein the magnitude and phase correction coefficients extend the effective bandwidth of the loudspeaker 
     
     
         14 . The apparatus of  claim 11 , wherein the input frame generator generates input frames that each have a degree of overlap with at least one preceding frame, and is configured to apply a constant-overlap-add windowing function to the input frames. 
     
     
         15 . The apparatus of  claim 11 , further comprising a volume control interface adapted to receive an indication of a selected one of a plurality of possible volume levels for the reproduction of the audio signal, and wherein the storage device has stored therein a plurality of sets of magnitude and phase correction coefficients, one for each possible volume level. 
     
     
         16 . The apparatus of  claim 15 , wherein, for all frequency bands in its scaling process, the multiplier is configured to use the set of magnitude and phase correction coefficients corresponding to the selected volume level. 
     
     
         17 . The apparatus of  claim 15 , wherein, for each frequency band in its scaling process, the multiplier is configured to use the set of magnitude and phase correction coefficients corresponding to an effective volume level, wherein the effective volume level is derived by scaling the selected volume level by the magnitude of the frequency band being scaled. 
     
     
         18 . The apparatus of  claim 11 , further comprising a dither generator configured to apply a degree of dither to each frequency band that is output by the multiplier prior to the creation of an output frame. 
     
     
         19 . The apparatus of  claim 11 , further comprising an analog to digital converter configured to convert the output audio stream to an output analog signal, an amplifier configured to amplify the output analog signal to provide an amplified signal. 
     
     
         20 . The audio processing apparatus of  claim 11 , forming part of any one of:
 a television;   a sound bar;   a personal media player docking station;   a tablet computer;   a mobile telephone;   a personal computer; or   a high-fidelity stereo system.